Efflux mediated by P-glycoprotein (P-gp) and multidrug resistance protein (MRP) expressed on the basolateral membrane of the RPE makes drug delivery to the posterior ocular chamber a challenging task. Currently, direct intravitreal administration is the most effective mode of drug delivery for back-of-the-eye diseases. This route of administration, however, is associated with a number of risks such as retinal detachment, secondary infections etc. The objective of this proposal is to elucidate whether topically applied (as eye drops) P-gp and/or MRP substrates and/or inhibitors can modulate the activity of the efflux proteins, P- gp and/or MRP, expressed on the retinal pigmented epithelium (RPE), and thus alter ocular pharmacokinetics of systemically/intravitreally co-administered substrates. This application is based on the hypothesis that topically applied P-gp/MRP substrates can interact with P-gp/MRP expressed on the basolateral membrane of the RPE and modulate their efflux activity. P-gp/MRP substrates, following topical instillation, may reach the sclera either by lateral diffusion along the corneal-scleral junction, or by permeating across the conjunctiva into the sclera. The sclera and the underlying choroid do not pose a significant barrier to drug diffusion. Thus, a fraction of the topically applied agent can penetrate to the RPE and potentially interact with and influence the efflux activity of P-gp/MRP expressed on the basolateral membrane of the RPE. Topically applied P-gp/MRP substrates could thus affect RPE's barrier properties with respect to P-gp/MRP substrates. Consequently, drug penetration into the back-of-the-eye from the systemic circulation could be significantly enhanced. Conversely, topically applied P-gp/MRP substrates may decrease elimination of intravitreally administered substrates, and could thus prolong their retention in the back-of-the eye ocular tissues. Preliminary studies indicate that the vitreal elimination half-life of intravitreally administered quinidine is increased almost 2.5-folds in the presence of topically co-administered erythromycin. The novel studies proposed in this application will delineate the effect of topically applied P-gp/MRP substrates on ocular pharmacokinetics of systemically or intravitreally administered substrates in an anesthetized rabbit model. Quinidine, a substrate of P-gp/MRP, and, whose interaction with RPE P-gp has been demonstrated, will be employed as a marker compound in these exploratory studies. Erythromycin and prednisolone, commonly employed topical agents and established substrates of P-gp and/or MRP, will be administered topically. Thus, this application seeks to evaluate the effect of topically administered erythromycin and prednisolone (3 doses each) on ocular pharmacokinetics of (a) intravitreally and (b) systemically administered quinidine. It is expected that based on the findings from this study a novel approach for localized modulation of the efflux activities of RPE's P-gp and/or MRP can be developed and utilized for back-of-the-eye drug delivery. /Relevance Efflux mediated by P-glycoprotein (P-gp) and multidrug resistance proteins (MRP) expressed on the retinal pigmented epithelium makes drug delivery to the posterior chamber ocular tissues, or back-of-the-eye, a challenging task. Currently, direct intravitreal administration is the preferred mode of drug delivery for back-of- the-eye diseases. This route of administration, however, is associated with a number of risks. The studies proposed in this application will explore a noninvasive strategy for localized inhibition of the efflux activities of P-gp and MRP, which could lead to the generation or maintenance of enhanced drug concentrations in back- of-the-eye ocular tissues, following systemic or intravitreal administration. Results from this research project may thus improve treatment outcomes for posterior chamber ocular diseases such as endophthalmitis, diabetic retinopathy, macular degeneration, retinoblastoma and other intraocular tumors. [unreadable] [unreadable]